Underground metal structures such as the underground casings of oil and gas wells and underground water tanks are subject to corrosion in localized areas due to electrical potential differences developed when the structure extends through different underground formations. The different formations generally each contain water having a different salt concentration, and therefore different potential differences are developed between the two sections of the structure in contact with the two formations. Electrons leave one of these sections, rendering this first area of this section anodic, flow through the structure and collect on the other of the sections, rendering this second area cathodic. The positive hydrogen ions then complete the circuit by gathering on the cathodic area through the formation.
As the electrons flow through the structure, an electrochemical process causes hydrogen atoms to form in the cathode area and iron from the casing to dissolve in the anode area. The iron is dissolved by the formation of iron ions. The hydrogen formed in the cathode area is removed by reaction with oxygen to form water or by the action of hydrogen consuming bacteria.
If the electron flow is permitted to continue, enough iron will be removed from the structure to corrode the structure and develop leaks.
Conventionally, this electrochemical corrosion in the anode area is prevented by connecting the negative terminal of a direct current source to the structure and connecting the positive terminal of the source to an anode buried in the earth adjacent to the well. If the direct current is appropriately applied, all points along the length of the structure will be cathodic with respect to the buried anode, the electrons will flow from the anode to the structure through the metallic path, and no corrosion of the structure occurs.
In accordance with conventional practice, soil resistivity measurements are conducted prior to the design of the anode bed to determine what is believed to be the best spot to install the anode bed. Where big differences in the soil resistivity are found, fixed value resistors have been installed in series with the individual anodes to control the respective anode discharge currents. However, this leads to unsafe conditions, especially in plant areas, due to the hazard associated with the heat generated from those resistors, which can be a source of ignition. The alternative is unbalanced individual anode currents.
Also, in accordance with conventional practice, water tank internal and external surfaces cannot be protected using a single rectifier, due to the difference in current requirements. The same problem applies to offshore metal structures protected with different types of anodes.
In coastal areas, cathodic protection systems protect the soil side of the structure as well as the submerged sections by connecting all the anodes to a common junction box. Multiple well casings connected to a common rectifier depend on the resistance of the negative cables to balance the current.
Accordingly, in view of the possibility of a large number of structures with differing potential requirements, the prior art has used an anode bed including a plurality of anodes of different sizes connected to a plurality of rectifiers for providing different amounts of cathodic protection current to these structures. This type of arrangement has proven to be expensive and cumbersome.
Moreover, this prior art arrangement can lead to errors in which incorrect amounts of current are drawn from the anodes.
FIG. 1 illustrates a conventional cathodic protection system including a junction box 10. An underground metal pipe 12 is provided at one position and an anode field 14, including a plurality of anodes 16, 18, is provided underground at another position.
A DC voltage source 20 is provided, having a positive terminal 22 and a negative terminal 24, with the junction box 10 positioned electrically between the positive terminal 22 of the voltage source 20 and the anode bed 16, 18. A respective cable 26, 28 extends from each anode 16, 18 to a corresponding terminal 36, 38 on the junction box 10, and a cable 30 extends from the positive terminal 22 of the voltage source 20 to a corresponding terminal 32 on the junction box 10. A cable 34 connects the negative terminal 24 of the voltage source 20 to the pipe 12.
One problem with this prior art structure is that the individual anode current outputs may be unbalanced depending on, for example, the different resistivities of the soil at different locations. This problem has been addressed in the prior art by either (a) disconnecting some of the anodes if they produce more current than specified by the manufacturer, or (b) having the high current output anodes consume first, with the remaining anodes sharing the additional load, thereby increasing the anode bed resistance and shortening the life of the anode bed.